Electric power generation control system and electric power generation control method for fuel cell

ABSTRACT

A power generation control system for a fuel cell, including a chargeable/dischargeable unit connected to a fuel cell, for being charged with electric power from the fuel cell and discharging electric power to a load, a target power computing unit for computing a target power to be generated by the fuel cell, a power-lowering request detection unit for detecting a power-lowering request to the fuel cell, an available power detection unit for detecting available power of the chargeable/dischargeable unit, a power extraction limiting unit for limiting electric power to be extracted from the fuel cell based on the power-lowering request detected by the power-lowering request detection unit and the available power detected by the available power detection unit, and a power extraction control unit for controlling electric power to be extracted from the fuel cell based on the target power and an output of the power extraction limiting unit.

TECHNICAL FIELD

The present invention relates to an electric power generation controlsystem for a fuel cell, for controlling electric power to be extractedfrom the fuel cell, and a related control method.

BACKGROUND ART

Fuel cells are electrochemical devices which convert the chemical energyof a chemical reaction directly into electrical energy. A typical fuelcell consists of an electrolyte membrane and anode and cathodecompartments sandwiching the electrolyte membrane therebetween, in whichfuel gas is fed continuously to the anode compartment, air is fedcontinuously to the cathode compartment, and oxygen from air andhydrogen contained in the fuel gas electrochemically reacts to generateelectric power. If gas shortage occurs in a local region in thecompartment, power output of the fuel cell drops, which may result inovercurrent causing cells to be damaged. It is required in control of afuel cell to have a function to detect the presence of such deterioratedoperating condition and recover the deteriorated operating condition, soas to preclude the cells from being damaged.

Japanese Patent Application Laid-Open Publication No. 6-243882 disclosesa control method for a fuel cell stack compartmentalized into aplurality of cell sections, in which output voltages of the cellsections are monitored and when the lowest voltage in the detected dropsbelow a predetermined value, the operation of the fuel cell stack isstopped for protecting the system regardless of the magnitude of load onthe fuel cell.

DISCLOSURE OF INVENTION

In the control method set forth above, if the detected voltage of acertain cell section drops below the predetermined value, the operationof the fuel cell is forced to stop even in a situation with theoperating condition thereof remaining in a slightly deteriorated degree.If operation of the fuel cell is stopped, a vehicle powered by the fuelcell can continue traveling only for a residual charge of a secondarybattery.

The present invention was made in the light of this problem. An objectof the present invention is to provide an electric power generationcontrol system for a fuel cell, which controls power extraction from thefuel cell and maintain the drive power, without causing any furtherdeterioration in the operating condition of the system.

An aspect of the present invention is a power generation control systemfor a fuel cell, comprising: a chargeable/dischargeable unit connectedto a fuel cell, for being charged with electric power from the fuel celland discharging electric power to a load; a target power computing unitfor computing a target power to be generated by the fuel cell; apower-lowering request detection unit for detecting a power-loweringrequest to the fuel cell; an available power detection unit fordetecting available power of the chargeable/dischargeable unit; a powerextraction limiting unit for limiting electric power to be extractedfrom the fuel cell based on the power-lowering request detected by thepower-lowering request detection unit and the available power detectedby the available power detection unit; and a power extraction controlunit for controlling electric power to be extracted from the fuel cellbased on the target power computed by the target power computing unitand an output of the power extraction limiting unit, wherein, as thepower-lowering request detection unit detects a power-lowering request,the power extraction limiting unit reduces electric power to beextracted from the fuel cell by an amount less than the available powerof the chargeable/dischargeable unit.

BRIEF DESCRIPTION OF DRAWINGS

The invention will now be described with reference to the accompanyingdrawings wherein:

FIG. 1 is a block diagram of a fuel cell system involving an electricpower generation control system for a fuel cell according to a firstembodiment of the present invention.

FIG. 2 is a control diagram of an electric power generation controlsystem according to first to fourth embodiments of the presentinvention.

FIG. 3 is a control flowchart illustrating a basic process flow of anelectric power generation control system of the first embodiment.

FIG. 4 is a graph showing the relationship between charge status of arechargeable battery and dischargeable power thereof.

FIG. 5 is a flowchart illustrating a basic process flow for providing apower-lowering request during control of the electric power generationcontrol system of the first embodiment.

FIGS. 6A to 6E are timing charts illustrating operation of the electricpower generation system of the first embodiment.

FIG. 7 is a block diagram of a fuel cell system involving an electricpower generation control system for a fuel cell according to second tofourth embodiments of the present invention.

FIG. 8 is a flowchart illustrating a basic process flow for providingthe power-lowering request during control of the electric powergeneration control system of the second embodiment.

FIGS. 9A to 9E are timing charts illustrating operation of the electricpower generation system of the second embodiment.

FIG. 10 is a flowchart illustrating a basic process flow for providingthe power-lowering request during control of the electric powergeneration control system of the third embodiment.

FIGS. 11A to 11E are timing charts illustrating operation of theelectric power generation system of the third embodiment.

FIG. 12 is a control flowchart illustrating a basic process flow of anelectric power generation control system of the fourth embodiment.

FIGS. 13A to 13E are timing charts illustrating operation of theelectric power generation system of the fourth embodiment.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments of the present invention will be explained below withreference to the drawings, wherein like members are designated by likereference characters.

An electric power generation control system for a fuel cell of the firstembodiment is involved in a fuel cell system S1 in FIG. 1, whichincludes a fuel tank 1, a compressor 2, a fuel cell 3, a powercontroller 4, a rechargeable battery (power rechargeable unit orsecondary battery) 5, a load (drive motor) 6, a motor 7, pressurecontrol valves 8, 9, and a controller 10.

The fuel cell 3 consists of a hydrogen electrode 3 a to be supplied withhydrogen gas as fuel gas, an air electrode 3 b to be supplied with airas oxidant gas, a high polymer electrolyte membrane M sandwiched betweenthe hydrogen electrode 3 a and the air electrode 3 b. Hydrogen (H₂)contained in the supplied hydrogen gas frees electrons at the hydrogenelectrode 3 a to form two protons (H⁺). The protons diffuse through theelectrolyte membrane M and at the air electrode 3 b react with theelectrons and oxygen (O₂) in the supplied air to form water (H₂O). Thisprovides an electric current in an external circuit between the hydrogenelectrode 3 a and the air electrode 3 b.

Hydrogen gas is fed from the fuel tank 1 to the hydrogen electrode 3 a.The pressure control valve 8 is provided on a fuel supply line 1 abetween the fuel tank 1 and the fuel cell 3 to control the flow rate andpressure at which hydrogen gas is supplied to the fuel cell 3.

In the meanwhile, air is compressed with the compressor 2 driven by themotor 7 and supplied through an air supply line 2 a to the air electrode3 b of the fuel cell 3. The flow rate and pressure at which air issupplied to the fuel cell 3 are adjusted by changing the number ofrevolutions of the motor 7 and a set pressure of the pressure controlvalve 9 provided on an air discharge line 2 b from an outlet of the airelectrode 3 b of the fuel cell 3.

The power controller 4 serves to allow electric power to be extractedfrom the fuel cell 3 and to be supplied to the load 6 and therechargeable battery 5.

The rechargeable battery 5 is connected between the fuel cell 3 and theload 6 in parallel thereto and operative to be charged when the load 6consumes electric power at a lower rate than that supplied from thepower controller 4 while discharging electric power when the load 6consumes electric power at a higher rate than that supplied from thepower controller 4.

The controller 10 computes target power TPW to be generated by the fuelcell 3, and adjusts set pressures PR1, PR2 of the pressure controlvalves 8, 9 and the revolution speed RN of the motor 7 so that electricpower is generated at a predetermined rate, while delivering extractionpower command CPW to the power controller 4. The controller 10 is alsoresponsive to an accelerator displacement value D1 and a vehicle speedD2 detected by sensors 11 to calculate demanded load, and applies anoperating command COP to the load 6 for controlling drive power of avehicle.

Further, although the power controller 4 is connected between the fuelcell 3 and the load 6 in FIG. 1, the power controller 4 may be connectedbetween the rechargeable battery 5 and the load 6 so as to controlelectric power to be supplied from the fuel cell 3.

In FIG. 2, the electric power generation system includes a target powercomputing unit 101, a power-lowering request detection unit 102, a powerextraction limiting unit 103, a fuel cell system control unit 104, apower extraction control unit 105, and a rechargeable-batteryavailable-power computing unit 106.

The target power computing unit 101 computes target power TPW to begenerated by the fuel cell 3. The power-lowering request detection unit102 detects a power-lowering request PLR for electric power to begenerated by the fuel cell 3. The rechargeable-battery available-powercomputing unit 106 computes available power APW to be outputted from therechargeable battery 5. The power extraction limiting unit 103 providescommands for limiting electric power to be extracted from the fuel cell3 from the target power TPW from the target power computing unit 101,the power-lowering request PLR from the power-lowering request detectionunit 102 and the available power APW from the rechargeable-batteryavailable-power computing unit 106. In response to the output from thepower extraction limiting unit 103, the fuel cell system control unit104 controls the fuel cell system S1, and the power extraction controlunit 105 controls electric power to be extracted from the fuel cell 3.

Now, a basic process flow to be executed in the fuel cell system S1 ofthe first embodiment will be described with reference to a flowchartshown in FIG. 3. The routine of the processes from “START” to “END” inthe flowchart are repeatedly executed every fixed time interval.

In step S1 in FIG. 3, the target power computing unit 101 computestarget power TPW for power generation by the fuel cell 3 from the datasuch as the accelerator displacement value D1, the vehicle speed D2 andpower demanded by the drive motor as the load 6.

In step S2, the rechargeable-battery available-power computing unit 106computes available power APW of the rechargeable battery 5. FIG. 4 showsan example of the relationship between a charge status CH % of therechargeable battery 5 and dischargeable power DPW thereof. Availablepower APW is set to be equal to a residual power obtained by subtractinga currently discharging electric power from the dischargeable power DPW.

In step S3, the power-lowering request detection unit 102 detects thepower-lowering request PLR to provide an output indicative of presenceor absence of this request.

In step S4, determination is made to find whether the power-loweringrequest PLR is provided, in response to the output indicative ofpresence or absence of the power-lowering request PLR provided in thepreceding step S3. If it is determined that no power-lowering requestPLR is present, process proceeds to step S5-3 where the extraction powerCPW is set to be equal to the target power TPW. If it is determined instep S4 that the power-lowering request PLR is present, process isrouted to step S5 where determination is made to find whether a limit isset on electric power to be generated in the preceding routine.

If it is determined in step S5 that no limit is set on electric power tobe generated, process proceeds to step S5-2 where the extraction powerCPW is set to be equal to a value obtained by subtracting the availablepower APW computed in step S2 from the target power TPW.

If it is determined in step S5 that a limit is set on electric power tobe generated, that means the power-lowering request PLR is still presentto further lower the extraction power CPW which has been limited in thepreceding routine by subtracting the available power APW of therechargeable battery 5 therefrom, the extraction power CPW is set to bea value obtained by subtracting the available power APW computed in stepS2 and a value α of predetermined amount from the target power TPW.

In step S6, various actuators such as the pressure control valves 8, 9and the motor 7 of the fuel cell system S1 are controlled in dependenceupon the extraction power command CPW computed in one of these stepsS5-1, S5-2 and S5-3.

In the first embodiment, the power-lowering request detection unit 102is provided with a voltage sensor 12 (cell voltage detection unit), asshown in FIG. 1, to detect the lowest voltage (the lowest cell voltageLCV) among cell voltages CVs of various unit cells or a plurality ofunit cell groups in the fuel cell 3, and is operative to determine thepresence of the power-lowering request PLR based on the detected lowestcell voltage LCV.

Next, detailed description will be made on processes for detecting thepower-lowering request PLR in step S3 following steps S1, S2 in theflowchart shown in FIG. 3 with reference to a flowchart of FIG. 5.

In step S3-0 in FIG. 5, the lowest cell voltage LCV of the unit cellsforming the fuel cell is detected.

In step S3-1, determination is made to find whether the limit is set onelectric power to be generated during preceding computing step. If it isdetermined that no limit is set on electric power to be generated,process is routed to step S3-3 where determination is made to findwhether the lowest cell voltage LCV is less than a threshold forrequesting to lower power extraction TFR. If it is determined that thelowest cell voltage LCV is less than the threshold for requesting tolower power extraction TFR, process is routed to step S3-6 where thecontroller sets that the power-lowering request is present.

On the contrary, if it is determined that the lowest cell voltage LCV isgreater than the threshold for requesting to lower power extraction TFR,process is routed to step S3-7 where the controller sets that thepower-lowering request is not present. Further, if it is determined instep S3-1 that the limit is set on electric power to be generated duringpreceding computing step, process is routed to S3-2 where determinationis made to find whether the lowest cell voltage LCV is less than athreshold for ending request to lower power extraction TFE. If it isdetermined that the lowest cell voltage LCV is less than the thresholdfor ending the request to lower power extraction TFE, process is routedto step S3-4 where it is determined that no cell voltage CV is stillrecovered with a need for further limiting electric power to beextracted, and the controller sets that the power-lowering request ispresent.

On the contrary, if it is determined that the lowest cell voltage LCV isgreater than the threshold for ending request to lower power extractionTFE, process is routed to step S3-5 where it is determined that the cellvoltage CV is already recovered with no need for limiting powerextraction, and the controller sets that no power-lowering request ispresent. Subsequently, processes in steps S4 to S6 shown in FIG. 3 areexecuted.

FIGS. 6A to 6E illustrate operations to be executed in the firstembodiment. FIG. 6A shows variation in the lowest cell voltage LCV interms of time. In FIG. 6A, an upper broken line designates the thresholdfor ending request to lower power extraction TFE, and a lower brokenline designates the threshold for requesting to lower power extractionTFR. FIG. 6B shows the presence of or the absence of the power-loweringrequest in terms of time. In FIG. 6B, a value “1” represents thepresence of the power-lowering request, and a value “0” represents theabsence of the power-lowering request. FIG. 6C shows variation infuel-cell extraction power CPW in terms of time. In FIG. 6C, a solidpolygonal line represents extraction power CPW to be extracted from thefuel cell, and a broken line represents target power TPW. FIG. 6D showsvariation in rechargeable battery power in terms of time. In FIG. 6D, asolid polygonal line represents actually discharged power, and a brokenline represents dischargeable electric power DPW to be discharged fromthe rechargeable battery. FIG. 6E shows variation in drive power interms of time. FIGS. 6A to 6E totally show the relationship among cellvoltage, the presence of or the absence of the power-lowering request,fuel cell extraction power, rechargeable battery power and drive power.

As shown in FIGS. 6A, 6B, when the lowest cell voltage LCV falls withthe elapse of time and reaches the threshold for requesting to lowerpower extraction TFR at time T1, the power-lowering request is provided.And as shown in FIGS. 6C, 6D, extraction power CPW is limited and therechargeable battery compensates electric power by the amount ofdecrement equivalent to resulting extraction power CPW. This decreasesthe load of the fuel cell, thereby permitting the lowest cell voltageLCV to be gradually restored. Then, as shown in FIGS. 6A, 6B, if thelowest cell voltage LCV reaches the threshold for ending request tolower power extraction TFE at time T2, the power-lowering request iscancelled. At the same time, as shown in FIGS. 6C, 6D, the limit onfuel-cell extraction power CPW is cancelled, thereby terminatingelectric power compensation from the rechargeable battery. During thisperiod, no adverse affect occurs in drive power to be delivered to avehicle as shown in FIG. 6E.

As will be appreciated from FIGS. 6A to 6E, the power-lowering requestis derived depending upon the value of the lowest cell voltage LCVwhereupon decreasing fuel-cell extraction power CPW by the valueequivalent to available power APW (which equals to dischargeable powerDPW in FIG. 6D) of the rechargeable battery enables the lowest cellvoltage LCV to be limited from continuous drop, caused by deteriorationin an operating condition of the fuel cell due to localized gasshortage, without causing any adverse affect on the drive power.

As set forth above, the first embodiment contemplates the provision ofthe power-lowering request detection unit 102 that detects the presenceof the power-lowering request regardless of the load demand that iscomputed based on the accelerator displacement value D1 and the vehiclespeed D2. Thus, when power-lowering request detection unit 102 detectsthe presence of the power-lowering request related to the fuel cell 3,due to the presence of the limited amount of extraction power CPWresulting from the fuel cell 3, to be limited, remains in a range withindischargeable electric power DPW of the rechargeable battery 5,extraction power CPW can be successfully controlled without invitingdeterioration in the operating condition of the fuel cell system S1while suppressing adverse affect on the drive power. That is, decreasingextraction power CPW from the fuel cell by the amount equivalent toavailable power APW of the rechargeable battery 5 enables to satisfy thepower-lowering request while suppressing adverse affect on the drivepower.

Further, by providing the power-lowering request in the presence of dropin the lowest cell voltage LCV, a further drop in the lowest cellvoltage LCV, resulting from deteriorated operating conditions of thefuel cell 3 due to the occurrence of localized gas shortage, can besuppressed.

Furthermore, due to a capability of detecting the recovery of the lowestcell voltage LCV and canceling the power-lowering request for therebypermitting extraction power CPW of the fuel cell 3 to be restored to itsoriginal state, the fuel cell system S1 can be rapidly restored to anormal electric power generation mode, minimizing the load to thesecondary battery.

Next, a second embodiment of the present invention is described withreference to a fuel cell system S2 for a vehicle involving an electricpower generation control system for a fuel cell shown in FIG. 7.

In the second-embodiment shown in FIG. 7, a gas delivery system includesan ejector 21, pressure sensors 21, 23, a purge valve 24, a hydrogenrecirculation line 25, a heat exchanger 26, a combustor 27, a coolantpassage 28, a pump 29 and temperature sensors 30, 31. In FIG. 7, thesame component parts as those of FIG. 1 bearing the same referencenumerals have the same functions.

The pressure control valve 9 controls the pressure at the air electrode3 b of the fuel cell 3 to a value depending upon the load in response toa pressure value PR3 detected by the pressure sensor 23 positioned atthe inlet of the air electrode 3 b of the fuel cell 3. The pressurecontrol valve 8 controls the pressure at the hydrogen electrode 3 a ofthe fuel cell 3 to a value depending upon the load in response to apressure value PR4 detected by the pressure sensor 22 positioned at theinlet of the hydrogen electrode 3 a of the fuel cell 3. The purge valve24 is located in an exhaust line 1 b extending from the hydrogenelectrode 3 a of the fuel cell 3 and normally kept closed. Upondetection of a drop in the cell voltage CV caused by water jamming inthe fuel cell 3, the purge valve 24 is opened to expel moisture contentwith hydrogen gas from the line to the outside.

Located downstream of the pressure control valve 9 and the purge valve24 is the combustor 27 in which exhaust air and purged exhaust hydrogengases join and are combusted. The temperature sensor 31 is mounted tothe combustor 27 to detect the temperature TE1 of combustion gas. Also,the hydrogen recirculation line 25 has one end coupled to the upstreamof the purge valve 24. The other end of the line 25 is coupled to a fuelsupply line 1 a downstream of the pressure control valve 8 via theejector 21. This allows a charge of hydrogen, that has not been fullyconsumed, to be fed to the hydrogen electrode 3 a of the fuel cell 3again for the purpose of maintaining a stoichiometric ratio (indicativeof supply flow rate/consumption flow rate) at a value greater than “1”to stabilize the cell voltage CV.

The coolant passage 28 serves as a passageway through which coolantflows to cool the body of the fuel cell 3. Provided on the coolantpassage 28 are the heat exchanger 26 and the pump 29 to flow coolant.The temperature sensor 30 detects a coolant temperature TE2 of the fuelcell 3.

In the second embodiment, the power-lowering request detection unitserves to detect the temperature TE1 of the combustor 27 shown in FIG. 7and makes determination, in response to the detected temperature TE1, tofind whether the power-lowering request is present. The basic processflow in control of the second embodiment is identical to that of theflowchart shown in FIG. 3 except for step S3 related to process for thepower-lowering request. Process for the power-lowering request isdescribed below with reference to a flowchart shown in FIG. 8.

In step S3-20 in FIG. 8, the temperature sensor 31 detects thetemperature TE1 of the combustor 27.

In step 3-21, determination is made to find whether the limit is set onelectric power to be generated during preceding computing step. If it isdetermined that no limit is set on electric power to be generated,process is routed to step S3-23 where determination is made to findwhether the temperature TE1 of the combustor 27 is greater than thethreshold for requesting to lower power extraction TFR. If it isdetermined that the temperature TE1 of the combustor 27 is greater thanthe threshold for requesting to lower power extraction TFR, process isrouted to step 3-26 where the controller sets that the power-loweringrequest is present.

In contrast, if it is determined that the temperature TE1 of thecombustor 27 is less than the threshold for requesting to lower powerextraction TFR, process is routed to step 3-27 where the controller setsthat the power-lowering request is absent. If it is determined in stepS3-2 1 that the limit is set on electric power to be generated duringpreceding computing step, process is routed to step S3-22 wheredetermination is made to find whether the temperature TE1 of thecombustor 27 is greater than the threshold for ending request to lowerpower extraction TFE. If it is determined that the temperature TE1 ofthe combustor 27 is greater than the threshold for ending request tolower power extraction TEE, process is routed to step S3-24 where it isdetermined that the temperature TE1 of the combustor 27 is notadequately lowered yet and there is a need for further limiting electricpower to be extracted whereupon the controller sets that thepower-lowering request is present.

In the meanwhile, if it is determined that the temperature TE1 of thecombustor 27 is less than the threshold for ending request to lowerpower extraction TFE, process is routed to step 3-25 where it isdetermined that the temperature TE1 of the combustor 27 is adequatelylowered and there is no need for further limiting electric power to beextracted whereupon the controller sets that the power-lowering requestis absent. Thereafter, processes in step S4 to S6 sown in FIG. 3 arerepeatedly executed in the same manner as those of the first embodiment.

FIGS. 9A to 9E illustrate operations to be executed in the secondembodiment. FIG. 9A shows variation in the temperature TE1 of thecombustor 27 in terms of time. In FIG. 9A, a lower broken linedesignates the threshold for ending request to lower power extractionTFE, and an upper broken line designates the threshold for requesting tolower power extraction TFR. FIG. 9B shows the presence of or the absenceof the power-lowering request in terms of time. In FIG. 9B, a value “1”represents the presence of power-lowering request, and a value “0”represents the absence of the power-lowering request. FIG. 9C showsvariation in fuel-cell extraction power CPW in terms of time. In FIG.9C, a solid polygonal line represents extraction power CPW to beextracted from the fuel cell, and a broken line represents target powerTPW. FIG. 9D shows variation in rechargeable battery power DPW in termsof time. In FIG. 9D, a solid polygonal line represents actuallydischarged power, and a broken line represents dischargeable electricpower DPW of the rechargeable battery. FIG. 9E shows variation in drivepower in terms of time. FIGS. 9A to 9E totally show the relationshipamong the temperature of the combustor 27, the presence of or theabsence of the power-lowering request, fuel cell extraction power,rechargeable battery power and drive power.

As shown in FIGS. 9A, 9B, when the temperature TE1 of the combustor 27increases with the elapse of time and reaches the threshold forrequesting to lower power extraction TFR at time T1′, power-loweringrequest is executed. And as shown in FIGS. 9C, 9D, extraction power CPWis limited and the rechargeable battery compensates electric power bythe amount of decrement equivalent to the resulting power CPW. Thisdecreases the load of the fuel cell, and the temperature TE1 of thecombustor 27 that has been increasing due to temporary shortage ofoxygen is gradually lowered. Then, as shown in FIGS. 9A, 9B, if thetemperature TE1 of the combustor 27 drops to threshold for endingrequest to lower power extraction TFE at time T2′, power-loweringrequest is cancelled. At the same time, as shown in FIGS. 9C, 9D, thelimit on fuel-cell extraction power CPW is cancelled, therebyterminating electric power compensation from the rechargeable battery.During this period, no adverse affect occurs in drive power to bedelivered to a vehicle as shown in FIG. 9E.

As will be appreciated from FIGS. 9A to 9E, extraction-power-loweringrequest is derived depending upon the temperature TE1 of the combustor27 whereupon decreasing fuel-cell extraction power CPW by the valueequivalent to available power APW of the rechargeable battery enablessuppression of temperature rise in the combustor 27 due to temporaryshortage of oxygen in air to be fed to the combustor 27 because of theoccurrence of control error or delay in response in the number ofrevolutions of a compressor drive motor, without causing any adverseaffect on the drive power.

As set forth above, the second embodiment contemplates to generateextraction-power-lowering request in the presence of an increase in thetemperature TE1 of the combustor 27 for exhaust hydrogen gas, therebysuppressing the occurrence of excessively high temperature rise in thecombustor 27 for exhaust hydrogen gas without sacrificing drive power.

Further, due to a capability of detecting the temperature drop in thecombustor 27 for exhaust hydrogen gas and cancelingextraction-power-lowering request to allow extraction power of the fuelcell 3 to be recovered to its original state, excessively hightemperature rise of the combustor 27 for exhaust hydrogen gas can besuppressed without sacrificing a quality of drive power and the fuelcell system S1 can be rapidly recovered to a normal electric powergeneration mode.

Next, a third embodiment of the present invention is described.

The third embodiment has a feature in that the power-lowering requestdetection unit permits the temperature sensor 30 shown in FIG. 7 todetect the temperature TE2 of coolant for cooling the fuel cell 3 toexecute determination for extraction-power-lowering request based on thedetected temperature TE2.

A basic sequence of control processes of the third embodiment isidentical to those of the flowchart shown in FIG. 3 except for processto provide power-lowering request in step S3. Process for providingpower-lowering request is described with reference to a flowchart shownin FIG. 10.

In step S3-40 in FIG. 10, the temperature sensor 30 detects thetemperature TE2 of coolant for cooling the fuel cell 3.

In step S3-41, determination is made to find whether the limit is set onelectric power to be generated during preceding computing step. If it isdetermined that no limit is set on electric power to be generated,process is routed to step S3-43 where determination is made to findwhether the coolant temperature TE2. of the fuel cell 3 is greater thanthe threshold for requesting to lower power extraction TFR. If it isdetermined that the coolant temperature TE2 of the fuel cell 3 isgreater than the threshold for requesting to lower power extraction TFR,process is routed to step S3-46 where the controller sets that thepower-lowering request is present. On the contrary, if it is determinedthat the coolant temperature TE2 of the fuel cell 3 is less than thethreshold for requesting to lower power extraction TFR, process isrouted to step S3-47 where the controller sets that the power-loweringrequest is absent.

In step S3-41, if it is determined that the limit is set on electricpower to be generated during preceding computing step, process is routedto step S3-42 where determination is made to find whether the coolanttemperature TE2 of the fuel cell 3 is greater than threshold for endingrequest to lower power extraction TEE. If it is determined that thecoolant temperature TE2 of the fuel cell 3 is greater than threshold forending request to lower power extraction TFE, process is routed to stepS3-44 where it is determined that the coolant temperature TE2 of thefuel cell 3 is not adequately lowered and there is a need for furtherlimiting electric power to be extracted whereupon the controller setsthat the power-lowering request is present.

If it is determined that the coolant temperature TE2 of the fuel cell 3is less than the threshold for ending request to lower power extractionTFE, operation is routed to step S3-45 where it is determined that thecoolant temperature TE2 of the fuel cell 3 is adequately lowered andthere is no need for further limiting electric power to be extractedwhereupon the controller sets that the power-lowering request is absent.Thereafter, processes in steps S4 to S6 shown in FIG. 3 are repeatedlyexecuted in the same manner as those of the first embodiment.

FIGS. 11A to 11E illustrate operations to be executed in the thirdembodiment. FIG. 11A shows variation in the coolant temperature TE2 ofthe fuel cell 3 in terms of time. In FIG. 11A, a lower broken linedesignates the threshold for ending request to lower power extractionTFE, and an upper broken line designates the threshold for requesting tolower power extraction TFR. FIG. 11B shows the presence of or theabsence of the power-lowering request in terms of time. In FIG. 11B, avalue “1” represents the presence of power-lowering request, and a value“0” represents the absence of the power-lowering request. FIG. 11C showsvariation in fuel-cell extraction power CPW in terms of time. In FIG.11C, a solid polygonal line represents extraction power CPW to beextracted from the fuel cell, and a broken line represents target powerTPW. FIG. 11D shows variation in rechargeable battery power in terms oftime. In FIG. 11D, a solid polygonal line represents actually dischargedelectric power, and a broken line represents dischargeable electricpower DPW of the rechargeable battery. FIG. 11E shows variation in drivepower in terms of time. FIGS. 11A to 11E totally show the relationshipamong the coolant temperature TE2 of the fuel cell 3, the presence of orthe absence of the power-lowering request, fuel cell extraction power,rechargeable battery power and drive power.

As shown in FIGS. 11A, 11B, when the coolant temperature TE2 of the fuelcell 3 increases with the elapse of time and reaches the threshold forrequesting to lower power extraction TFR at time T1″, the power-loweringrequest is executed. And as shown in FIGS. 11C, 11D, extraction powerCPW is limited and the rechargeable battery compensates electric powerby the amount of decrement equivalent to the resulting extraction powerCPW. This decreases the load of the fuel cell, and the coolingtemperature TE2 of the fuel cell 3 gradually decreases. Then, as shownin FIGS. 11A, 11B, if the coolant temperature TE2 of the fuel cell 3drops to the threshold for ending request to lower power extraction TFEat time T2″, the power-lowering request is cancelled. At the same time,as shown in FIGS. 11C, 11D, the limit on fuel-cell extraction power CPWis cancelled, thereby terminating power compensation to be made by therechargeable battery. During this period, no adverse affect occurs indrive power of the vehicle as shown in FIG. 11E.

As set forth above, the third embodiment contemplates to allow theextraction-power-lowering request to be provided based on an increase inthe coolant temperature TE2 that has correlation with the celltemperature of the fuel cell 3 so as to decrease extraction power CPW ofthe fuel cell 3 by an amount equal to available power APW of therechargeable battery 5. This suppresses the fuel cell 3 from excessivelyhigh temperature rise and deterioration in the fuel cell 3 can beavoided without causing adverse affect on drive power. Also, thepower-lowering request detection unit may be provided with thetemperature sensor 32 for directly measuring the cell temperature TE3 ofcells forming the fuel cell 3 to allow determination whether to providethe power-lowering request based on the resulting output.

Further, by detecting the temperature drop in the fuel cell 3 andcanceling the extraction-power-lowering request so as to allowextraction power of the fuel cell 3 to be recovered to an initialstatus, excessively high temperature rise of the fuel cell 3 can beavoided, and the fuel cell can be rapidly recovered to an initial normalelectric power generation mode while suppressing an adverse affect ondrive power, enabling reduction in load of the rechargeable battery at aminimal.

Next, a fourth embodiment of the present invention is described.

The fourth embodiment has a feature in that the power extractionlimiting unit is so arranged as to continuously vary power extraction(in a ramp form) between target power TPW and available power APW of therechargeable battery. Other features of the fourth embodiment aresimilar to those of the first to third embodiments and the fourthembodiment has the same structure as those shown in FIGS. 1, 3 and 7.

Next, a basic process flow of the fourth embodiment is described belowwith reference to a flowchart of FIG. 12.

Processes in steps S1 to S4 and step S6 in FIG. 12 are identical tothose shown in FIG. 3. In respect of process in step S3, process shownin either FIGS. 5, 8 or FIG. 10 may be applied.

As a result of determination in step S4, if no power-lowering request isfound, process is routed to step S4-2 where process is executed suchthat extraction power CPW is set to be greater than previous extractionpower, resulting from preceding computing step, by a predetermined valueof β. However, the maximum value of extraction power CPW is set to beequal to target power TPW, and no power greater than target power TPW isextracted.

On the contrary, if it is determined in step S4 that the power-loweringrequest is present, process is routed to step S4-1 where process isexecuted such that extraction power CPW is set to be less than previousextraction power CPW resulting from preceding computation by apredetermined value of γ. However, the minimum value of extraction powerCPW is treated to be equal to a product where available power APWresulting from computation in step S2 is subtracted from target powerTPW.

FIGS. 13A to 13E illustrate operations to be executed in the fourthembodiment. The operating sequence shown in FIG. 8 is applied to thefourth embodiment as the method of providing detecting power-loweringrequest in respect of step S3. FIG. 13A shows variation in thetemperature TE1 of the combustor 27 in terms of time. In FIG. 13A, alower broken line designates the threshold for ending request to lowerpower extraction TFE, and an upper broken line designates the thresholdfor requesting to lower power extraction TFR. FIG. 13B shows thepresence of or the absence of the power-lowering request in terms oftime. In FIG. 13B, a value “1” represents the presence of power-loweringrequest, and a value “0” represents the absence of the power-loweringrequest. FIG. 13C shows variation in fuel-cell extraction power CPW interms of time. In FIG. 13C, a solid polygonal line represents extractionpower CPW to be extracted from the fuel cell, and a broken linerepresents target power TPW. FIG. 13D shows variation in rechargeablebattery power in terms of time. In FIG. 13D, a solid polygonal linerepresents actually discharged electric power, and a broken linerepresents dischargeable electric power DPW of the rechargeable battery.FIG. 13E shows variation in drive power in terms of time. FIGS. 13A to13E totally show the relationship among the temperature TE1 of thecombustor 27, the presence of or the absence of the power-loweringrequest, fuel cell extraction power, rechargeable battery power anddrive power.

As shown in FIGS. 13A, 13B, when the temperature TE1 of the combustor 27increases with the elapse of time and reaches the threshold forrequesting to lower power extraction TFR at time T1′″, thepower-lowering request is executed. And as shown in FIGS. 13C, 13D,extraction power CPW is limited and the rechargeable battery compensateselectric power by the amount of decrement equivalent to the resultingextraction power CPW. This decreases the load of the fuel cell, and thetemperature TE1 of the combustor 27 gradually decreases. Then, as shownin FIGS. 13A, 13B, if the temperature TE1 of the combustor 27 drops tothe threshold for ending request to lower power extraction TFE at timeT2′″, the power-lowering request is cancelled. At the same time, asshown in FIGS. 13C, 13D, the limit on fuel-cell extraction power CPW iscancelled, thereby terminating electric power to be compensated from therechargeable battery. During this period, no adverse affect occurs indrive power of the vehicle as shown in FIG. 13E.

As will be appreciated from FIGS. 13A, 13B, with the fourth embodiment,by continuously lowering fuel-cell extraction power CPW to a value equalto available power APW of the rechargeable battery in dependence uponthe extraction-power-lowering request, a capability of dischargingrechargeable battery at a minimal enables to satisfy a demanded loadwithout an adverse affect on drive power.

The present disclosure relates to subject matter contained in JapanesePatent Application No. 2003-106495, filed on Apr. 10, 2003, thedisclosure of which is expressly incorporated herein by reference in itsentirety.

The preferred embodiments described herein are illustrative and notrestrictive, and the invention may be practiced or embodied in otherways without departing from the essential character thereof. The scopeof the invention being indicated by the claims, and all variations whichcome within the meaning of claims are intended to be embraced herein.

INDUSTRIAL APPLICABILITY

In the present invention, the power-lowering request detection unit 102is provided which detects power-lowering request of the fuel cell 3regardless of the demanded load whereby in the presence of thepower-lowering request, extraction power to be derived from the fuelcell 3 is limited, enabling the operating condition of the fuel cell 3to be prevented from further deterioration caused by excessive amount ofextraction power. Also, if the degree of deterioration in the operatingcondition of the fuel cell 3 remains little, the fuel cell system is notinterrupted in operation to enable power output to be continuouslyextracted.

1. A power generation control system for a fuel cell, comprising: achargeable/dischargeable unit connected to the fuel cell, for beingcharged with electric power from the fuel cell and discharging electricpower to a load; a target power computing unit for computing a targetpower to be generated by the fuel cell; a power-lowering requestdetection unit for detecting a power-lowering request to the fuel cell,comprising a cell voltage detection unit for detecting a cell voltage ofthe fuel cell, wherein the power-lowering request detection unitdetermines whether or not the power-lowering request is present based onthe cell voltage detected by the cell voltage detection unit; anavailable power detection unit for detecting available power of thechargeable/dischargeable unit; a power extraction limiting unit forlimiting electric power to be extracted from the fuel cell based on thepower-lowering request detected by the power-lowering request detectionunit and the available power detected by the available power detectionunit; and a power extraction control unit for controlling electric powerto be extracted from the fuel cell based on the target power computed bythe target power computing unit and an output of the power extractionlimiting unit, wherein, as the power-lowering request detection unitdetects the power-lowering request, the power extraction limiting unitreduces electric power to be extracted from the fuel cell by an amountless than the available power of the chargeable/dischargeable unit. 2.The power generation control system for a fuel cell according to claim1, wherein, as the power-lowering request detection unit detects anotherpower-lowering request, the power extraction limiting unit furtherreduces electric power to be extracted from the fuel cell.
 3. The powergeneration control system for the fuel cell according to claim 1,wherein the power extraction limiting unit keeps the electric power tobe extracted from the fuel cell reduced until the cell voltage detectedby the cell voltage detection unit is recovered to a predeterminedvoltage.
 4. A power generation control system for a fuel cell,comprising: a chargeable/dischargeable unit connected to the fuel cell,for being charged with electric power from the fuel cell and dischargingelectric power to a load; a target power computing unit for computing atarget power to be generated by the fuel cell; a power-lowering requestdetection unit for detecting a power-lowering request to the fuel cell,comprising a temperature detection unit for detecting a cell temperatureof the fuel cell, wherein the power-lowering request detection unitdetermines whether or not the power-lowering request is present based onthe cell temperature detected by the temperature detection unit; anavailable power detection unit for detecting available power of thechargeable/dischargeable unit; a power extraction limiting unit for1imiting electric power to be extracted from the fuel cell based on thepower-lowering request detected by the power-lowering request detectionunit and the available power detected by the available power detectionunit; and a power extraction control unit for controlling electric powerto be extracted from the fuel cell based on the target power computed bythe target power computing unit and an output of the power extractionlimiting unit, wherein, as the power-lowering request detection unitdetects the power-lowering request, the power extraction limiting unitreduces electric power to be extracted from the fuel cell by an amountless than the available power of the chargeable/dischargeable unit. 5.The power generation control system for the fuel cell according to claim4, wherein the power extraction limiting unit keeps the electric powerto be extracted from the fuel cell reduced until the cell temperaturedetected by the temperature detection unit drops below a predeterminedtemperature.
 6. The power generation control system for the fuel cellaccording to claim 1, wherein the power-lowering request detection unitcomprises: a purge gas combustor; and a combustor temperature detectionunit for detecting a temperature of the purge gas combustor, wherein thepower-lowering request is provided based on the temperature detected bythe combustor temperature detection unit.
 7. The power generationcontrol system for the fuel cell according to claim 6, wherein the powerextraction limiting unit keeps the electric power to be extracted fromthe fuel cell reduced until the temperature detected by the combustortemperature detection unit drops below a predetermined temperature. 8.The power generation control system for the fuel cell according to claim2, wherein the power extraction limiting unit increases the amount ofreduction in electric power to be extracted from the fuel cell to theavailable power of the chargeable/dischargeable unit by a predeterminedamount while the power-lowering request detection unit detects thepower-lowering request, and the power extraction limiting unit decreasesthe amount of reduction in electric power to be extracted from the fuelcell by another predetermined amount while the power-lowering requestdetection unit detects no power-lowering request.
 9. A method ofcontrolling electric power to be generated by a fuel cell, the methodcomprising: providing a chargeable/dischargeable unit for being chargedwith electric power from the fuel cell and discharging electric power toa load; computing target power to be generated by the fuel cell;detecting a power-lowering request to the fuel cell by detecting a cellvoltage of the fuel cell and determining whether or not thepower-lowering request of the fuel cell is present based on the detectedcell voltage; detecting available power of the chargeable/dischargeableunit; limiting electric power to be extracted from the fuel cell basedon the detected power-lowering request and the detected available power;and controlling the electric power to be extracted from the fuel cellbased on the computed target power and the action of limiting electricpower to be extracted, wherein, as the power-lowering request isdetected, the electric power to be extracted from the fuel cell isreduced by an amount less than the available power of thechargeable/dischargeable unit.
 10. A method of controlling electricpower to be generated by a fuel cell, the method comprising: providing achargeable/dischargeable unit for being charged with electric power fromthe fuel cell and discharging electric power to a load; computing targetpower to be generated by the fuel cell; detecting a power-loweringrequest to the fuel cell by detecting a cell temperature of the fuelcell and determining whether or not the power-lowering request to thefuel cell is present based on the detected cell temperature; detectingavailable power of the chargeable/dischargeable unit; limiting electricpower to be extracted from the fuel cell based on the detectedpower-lowering request and the detected available power; and controllingthe electric power to be extracted from the fuel cell based on thecomputed target power and the action of limiting electric power to beextracted, wherein, as the power-lowering request is detected, theelectric power to be extracted from the fuel cell is reduced by anamount less than the available power of the chargeable/dischargeableunit.